Flexible polysiloxane resins are useful spin-on dielectric and encapsulation materials. They are usually prepared by co-hydrolysis and subsequent condensation reaction between methyltrialkoxy-silane and other alkoxysilanes. They are used in semiconductor industry as insulation and planarising materials. Normally, they are spin-casted on a silicon wafer as polymer-solvent solutions, followed by soft bake at 100 to 150° C. to remove the solvent and final cure at 200 to 400° C., where remaining silanols thermally condense to form a cross-linked, insoluble film.
Methyltrialkoxysilane alone gives polymer with cracking threshold of only to 2 μm so, as such, it is unsuitable for thick films. A common co-monomer that is used to increase cracking threshold is diphenylsilanediol. However, compared to methyltrialkoxysilanes, diphenylsilanediol has low reactivity for which reason it is not easily co-polymerized into the polymer matrix. At normal hydrolysis conditions some of it remains non-reactive, and said monomer tends to crystallize out from the solution. If final spin-on solution contains unreacted diphenylsilanediol, some of it will fume out from the polymer film during the bake, causing a risk of particle contamination of process equipment.
There are some methods disclosed in the literature describing how to force diphenylsilanediol and other silanols of low reactivity to react into the polymer matrix. One way is to increase reaction temperature to over 100° C. with or without the use of catalysts such as aluminum, zirconium, tin or titanium catalysts (references 1 and 2). This, however, prevents the use of heat sensitive co-monomers in the polymer. Metal catalysts are difficult or even impossible to remove from the polymer solution. This severely limits the use of such polymers in semiconductor applications, where low metal contamination, often below 100 ppb total metals is required.
Another way of achieving forced incorporation of diphenylsilanediol into the polymer is to react it with chlorosilanes, such as methyltrichlorosilane, in the presence of an amine base as hydrogen chloride scavenger (reference 3). Unfortunately, chlorosilanes are very corrosive to laboratory and production equipment. Also the by-product, an amine-hydrochloride salt, needs to be removed from the reaction mixture by filtration, which is not readily achieved with moisture sensitive chlorosilanes.
One major problem in using diphenylsilanediol directly in materials for the semiconductor industry is that commercial grades of diphenylsilanediol contain metal impurities, sometimes even in the form of tiny rust particles. Being a solid material, it cannot be purified simply by distillation. This is a common problem with many silanol containing silane monomers with high or no boiling point combined with high melting points.
U.S. Pat. No. 3,122,579 discloses a method of preparing silane monomers in which an acetoxy- or chloro-modified monomer is reacted with silanol (Si−OH). The latter reaction causes a hydrochloro precipitate which is difficult to process, whereas the first reaction gives rise to acetoxy-silanes which are difficult to hydrolyze because there is then formed acetic acid in equivalent amounts. For this reason, acetoxy silanes are rarely used for polymerization; they are primarily suitable for RTV-cured silicone materials as a cross-linking agent.
GB Patent Specification No. 1139423A discloses a method of preparing siloxanes by reacting hydridosilanes with hydroxysilanes in the presence of an amine catalyst. Hydridosilanes are expensive chemicals and in the reactions described in the document hydrogen, an explosive gas, is formed which causes a safety risk when operating the process on an industrial scale.